1. Field of the Invention
The present invention relates to a method and a system for determining a linear speed of an object, in particular in a Cartesian frame of reference.
2. History of the Related Art
Linear speed computation could benefit applications such as self-guided vehicles (at 2 in FIG. 1), mobile robotics, smart vision systems, and body movement control for therapy (at 3 in FIG. 1) and entertainment (at 4 in FIG. 1). In general, the measurement of linear speed from optical information is useful to technical applications because it operates in the frame of reference of translating movements.
Moreover, linear speed can be used to compute the distance traveled. This requires multiplying the linear speed signal by the appropriate time interval.
The passive measurement of the linear speed from a visual scene relies on the light distributions generated by a translation. It is a technically challenging task mainly because a wide-field scene moving at constant translational speed projects on an image plane as a heterogeneous field of apparent speeds that depends on the distribution of spatial frequencies (FIG. 2). This field is called optical flow, and is what the existing visual speed sensors measure.
Each velocity vector of the optical flow field is a projection of the translational speed. These vectors can thus render the translational speed by means of a spherical-to-cartesian transformation. This requires, besides the intensity, direction and location of the vector in the spherical field, also the distance between the sensing entity and the projected moving object. This principle is applied in the so-called “V/H” sensors. These have commonly a narrow field of view, pointed perpendicularly to the direction of translation.
Unfortunately, the computation of speed using the V/H sensor is not as robust as it would be desirable in terms of accuracy.